We have studied equilibration with respect to isospin degree of freedom in four 96 mass systems Ru-96 + Ru-96, Ru-96 + Zr-96, Zr-96 + Ru-96, and Zr-96 + Zr-96 at 100A MeV and 400A MeV with isospin-dependent quantum molecular dynamics. We propose that the neutron-proton differential rapidity distribution is a sensitive probe to the degree of equilibration with respect to the isospin degree of freedom. By analyzing the average N/Z ratio of emitted nucleons, light charged particles, and intermediate mass fragments (IMF), it is found that there exist memory effect in multi fragmentation process. The average N/Z ratio of IMF reduces largely as beam energy increases from 100A MeV to 400A MeV that may result from the change of the behavior of the isotope distribution of IMF charges. The isotope distribution of IMF charges does also show certain memory effect at 100A MeV case but not at 400A MeV case.